As a positive electrode active material for a lithium secondary battery, LiCoO2, LiMn2O4, LiNixCoyMnzO2, and the like, have been mainly used. However, in accordance with the development of a middle and large-sized battery (hybrid electric vehicle (HEV), plug-in hybrid electric vehicle (PHEV), electric vehicle (EV)), a problem such as safety of the battery has been in the spotlight. In the case of the positive electrode active materials currently commercialized, since the positive electrode active material is expensive or there is a problem in terms of safety, or the like, research into a new positive electrode active material has been conducted.
Among various candidate materials, research into an olivine type LiMPO4 (M=Fe, Mn, Co, and Ni) positive electrode active material that is economical and safe has been actively conducted. Particularly, LiFePO4 has been currently developed as an energy source of a hybrid vehicle and an electric vehicle by allowing LiFePO4 to have a particle size of nano scale and coating LiFePO4 with carbon in order to solve a problem of low conductivity. (See Japanese Patent Laid-Open Nos. 2002-015735 and 2004-259470)
However, in the case of LiFePO4, since an average operation voltage is about 3.5V, there is a problem that energy density is lower than those of other positive electrode active materials. The energy density is low, which means that the number of cells of a battery in a limited volume may be insufficient as compared with the required number. That is, in order to have the same capacity, the volume should be increased. In order to overcome this problem, research into Mn3+/4+ (4.1V) having a discharge voltage higher than Fe2+/3+ (3.5 V) has been conducted.